Axonal, dendritic, or synaptic structures were identified by immunohistochemistry in developing neuronal cultures. In cultures where electrical activity was suppressed by chronic exposure to tetanus toxin or tetrodotoxin, synaptic structures were more abundant than in active cultures. Techniques are being developed for the intracellular injection of dyes into neurons in high density (maximally active) and toxin-exposed (electrically inactive) cultures in order to study dendrite development under these conditions during the first two weeks in culture.